Add support to multiple targets

This patch add support to use multiple targets in the same environment.

ICECC_VERSION variable will look like <native_filename>(,<platform>:<cross_compiler_filename>(=<target>))*

Where the target is defined by the cross-compiler prefix.

For example:

ICECC_VERSION=/work/4f7150ce1196b090fb4fb1f8a2891fbf.tar.gz,/android/arm-eabi-5128040f5d6b69ed422a69e091fc1f32.tar.gz=arm-eabi,/android/arm-linux-androideabi-ff0a26c22071e4e5430c9d31ad48a11d.tar.gz=arm-linux-androideabi,/android/i686-linux-e6f68fa6e0464ca83db17d93d5e9d0cb.tar.gz=i686-linux,/android/x86_64-linux-7ec6b4ceb76561beb1e154b848fd045c.tar.gz=x86_64-linux

For the compiler arm-eabi-gcc, icecc will look for the
tarball file with the target arm-eabi in the ICECC_VERSION var
because arm-eabi is the prefix of the cross-compile.

Signed-off-by: Ragner Magalhaes <ragner.magalhaes@gmail.com>
Doc for Cross-Compiling for multiple targets

Signed-off-by: Ragner Magalhaes <ragner.magalhaes@gmail.com>